Electron Paramagnetic Resonance (EPR) provides a sensitive means of detecting and quantitating free radical species. Conventional constant wave (CW) EPR has provided insight into the basic chemistry of free radical reactions and has most recently been increasingly used to probe the intricacies of biological intermediates. At this time, although EPR is vastly more sensitive than nuclear magnetic resonance, its use as a biological tool as been limited by both the frequency used--routinely greater 1 MHz--and the paucity of detectable signal. The Radiation oncology Branch is involved in the development of both a pulse wave (PW) EPR spectrometer and PW-EPR imaging. Because the electron decay are in the order of microseconds as compared to the vastly longer nuclear relaxation times, the use of nanosecond PW-EPR with extremely rapid signal averaging and processing offers the possibly of surmounting the previous limitations imposed by limitations impose signal. Likewise, the previous limitations imposed by CW-EPR MHz frequencies need not remain an impasse to EPR based imaging when pulse field and tenths of MHz frequencies and gradient profiling is employed. The project is multi-disciplinary requiring state of the art electronic, computation, gradient design, etc. The project is designed to create a prototypical PW-EPR instrument for in vitro biologic studies and to establish the foundational framework to demonstrate the application of PW-EPR to in vivo imaging. Hopefully, it will ultimately yield insight into cellular events that occur at the microsecond time scales and should because of the inherent physics of rapid free induction decay of excited electrons yield practically real time imaging with voxel resolution not attainable by conventional nuclear magnetic resonance based magnetic resonance imaging.